Coaxial transmission line spacing assembly



March 18, 1952 L, BONDON 2,589,328

COAXIAL TRANSMISSION LINE SPACING ASSEMBLY Filed Nov. 6, 1950 IN VENTOR kw/s A. Bawoo/v ATTORNEYS Patented Mar. 18, 1952 UNITED STATES PATENT OFFICE COAXIAL TRANSMISSION LINE SPACING ASSEMBLY Lewis A. Bondon, Arlington, N. J.

Application November 6, 1950, Serial No. 194,307

8 Claims. 1 This invention relates to electrical coaxia. transmission lines and has more particular ref erence to insulating assemblies for maintaining the conductors of such lines in concentric rela ion.

Conventional forms of coaxial transmission lines usually comprise a tubular inner conductor disposed in a tubular outer conductor and spaced concentrically therefrom by uniformly spaced insulators between inner are outer conductors. With such a known construction there are relatively abrupt changes in the characteristic impedance of the line where the insulators occur and electrical reflection efiects accordingly occur at these places.

It has been proposed to overcome this defect and make the characteristic impedance of such a line uniform by providing a change in diameter of one or both of the conductors at the points of engagement with the conductors by the insulators. Such constructions, while compensating for the change in impedance at the insulatorado not provide the electrical transparency required for truly broad band transmission performance. This is due to the fact that the deformationof the conductors to provide the change in diameter necessary to compensate for impedance produces a residual capacitive reaction at each insulator, causing electrical discontinuities which did not exist in the line before being deformed.

One object of the present invention is to provide a coaxial transmission line comprising an inner conductor disposed in a tubular outer conductor and spaced therefrom by insulating' assemblies, including radially disposed insulating pins, so constructed and arranged as to overcome the aforesaid diiliculties so that the transmission line will provide uniform signal transmission through any portion of its length and should not produce harmful reflections due to their insertion at any service up to the cutoif frequency of the line.

Another object of the present invention is to provide a transmission line, as above characterized, in which the effective diameter of the' inner conductor is suitably reduced to reduce the capacity of the inner conductor at each insulating assembly in order to keep the characteristic impedance of the line constant.

Another object of the present inventionis to provide a transmission line, as above characterized, wherein an air gap is formed around the inner end of each of the insulator spacingfpins at the point where they engage the effectively reduced diameter portion of the inner conductor to provide an inductance in series with the A further and more specific object of the present invention is to provide a transmission line,

' as above characterized, wherein the inner conductor is a circular tubular conductor, and wherein the insulating assemblies each comprises a reactance or X-slug slip fitted within the inner conductor and formed as a metallic cylindrical member having a wide, shallow, circumferential channel formed intermediate its ends and analrow, circumferential channel formed on the bottom surface of the wide channel intermediate its ends; and a diametrically opposed pair of cylindrical insulator pins' extending through the walls of the inner conductor with their inner ends saddle fitted within the narrow channel on the X-slug and their outer ends engaging the inner surface of the outer conductor; with the dimensions of the channels being so proportioned that the transmission line is electrically "transparent," thereby permitting it to provide truly broad band performance. I

I Other objects and advantages of the invention will be apparent in the following specification, when considered together with the accompanying drawings, wherein:

Fig. 1 is an elevational view with parts broken away, of a coaxial transmission line constructed in accordance with the present invention;

Fig. 2 is a vertical sectional view taken on the line 2-2 of Fig. 1; r

Fig. 3 is an enlarged perspective View of the X-slug shown in Fig. l;

Fig. 4 is an end view of one of the insulators shown in Figs. 1 and 2; and

Fig. 5 is an elevational view of the insulator shown in Fig. 4. v

The present invention is an improvement over that disclosed in my Patent No. 2,436,284, issued on February 1'7, 1948, for Coaxial. Transmission Line.

The coaxial transmission line disclosed in the aforesaid patent, while giving greatly improved line performance, permitting its use at frequencies beyond the range of the conventional air dielectric coaxial transmission line, does not provide truly broad band line performance, because the insulator structure used to support the inner conductor is not electrically transparent" to any radio frequency waves within the pass band of operation.

In general, the present invention provides a coaxial transmission line in which the construction of the insulator assemblies is designed to compensate not only for the change in the characteristic impedance, but also for any other flux line discontinuities caused by the design of the assemblage structure, where the insulator structure becomes electrically transparent" and provides truly broad band line performance.

struction and, as shown, each comprises a cylindrical reactance or X-slug I2, made of conducting material and having a circumferential channel or groove I3 formed thereon intermediate its ends; and a pair of diametrically opposed insulator spacing members it extending through diametrically opposed openings i5 inthe wall of the inner conductor with their inner ends saddle fitted within the cylindrical groove 13 and their outer ends engaging theinner surface of the outer conductor.

The X -slug I2 is preferably made of the same material as the inner conductor and is formed 'to the shape best shown in Fig.3. As there shown, the X-slug is in the form of a short cylindrical member having a wide circumferential channel or groove [6 formed in its outer surface intermediate its ends, providing shoulders IT at each end. The shoulders I! have ,end walls perpendicular to the longitudinal" axis of the member. The narrow circumferential channel forgroove i3 engaged by the insulators leis formed intermediate the ends of the channel It. The channels I3 and I6 are rectangular'in cross section and theirside walls are perpendicular to the axis'of the X-slug. The X-slug 2:15pm- Vided .Witha longitudinal slot [8 which gives the memberisufiicientjresilience to permit it toflbe inserted in theinner conductor with .the external surface of the shoulders I! tightly engaging the inner surfaceof the conductor.

Each of the insulator pins M is identical in construction and maybe of any suitable shape; however, in the particular embodiment thereof "illustrated, is shown as a cylindrical member having one end wedge-shaped'fas indicated at [9, and the other end made concavaas shown at 20,,with a surface curvaturethesame as the outer surface curvature of the channel portion 53 of the X-slug. The insulators Hi are preferably made of a compressible plastic organic insulating material such as .polytetrafiuorethylene.

Inassembling the transmission line, the X- ,slugs'arepositioned to bring their grooves 13 in alignment with their corresponding diametrically opposed-holes 15 in the inner conductor. Then, the insulator-pins l4 are'inserted through the-holes I5in the-inner conductor and pressed .into .a.tight saddle fit within the grooves l3 on the X-slugs which hold the inner ends of the ,pins in place (Fig. 1). ductor, withthe spacing assemblies in place. is press-fitted into the outer conductontheplaS- .tic material of which theiinsulator pins Mare made being slightly compressed to make anin- .timate physical fit with .the inner surface of the .outer conductor. To facilitate the insertionof the pin members M into theouter conductor, the longitudinal ends or the wedge- ..shaped ends lfilmay be beveled, as indicated .at 2|.

.The insulating ,assemblies are longitudinally spaced at any desired intervals and be dis posed on the innerconductor in any desired angu- Then, the inner ,con-' 4 lar relation. In the particular embodiment of the invention illustrated, they are shown as being angularly disposed relative to adjacent assemblies. The circumferential channels i3 on the X-slugs M are cut to a depth sufficient to provide a characteristic impedance at each insulator equal to the characteristic impedance of the transmission line in the air space portions between these insulators, or, in other words, to accomplish impedance compensation at the points of support of the inner conductor. The channel [3 of the X-slug produces the same effect as undercutting the inner conductor an amount equal to the dimension of the channel,

to reduce its diameter. The width of the circu'mferential channel 56 in each of the X-slugs is made larger than the diameter of the insulator pins to provide an air gap between the vertical side walls of the insulators and the vertical side walls of the channel. The air gap provides a series inductive reactance which cancels out the residual capacitive reactance of the insulator .which causes distortion of flux lines distributed .about this local. particular constructioi'i shown in Fig. l, is very The interface capacity of the small, due to the physical relationships of the assembly, as shown, and, therefore, the value. of inductance required for cancellation of this vcapacitive reactance is very small.

It willv be seen that with the construction illustrated and above described, it is possible, by suitably. proportioning the X-slugs, to decrease the capacitance of the innerjconductor at'each .insulator in order to keep the characteristic in;-

pedance'constant. Also, "the interface capacitive reacta'nce, dueto. distortion of flux lines .at the assembly localand remaining afteriinpedance. compensation has been accomplished, is

.cancelled by introduction. of a series inductance asprovided byth'e air gap between thevertical side walls' of the channel. formed on the X-s'lug andthe vertical sidewalls of the insulators, there- .by accomplishing a vcoaxial transmission line .which is transparent to RF waves, throughout the frequency range of the size of the line in use.

For example, a l%" electrical coaxial transmission linev as already supplied for commercial and military service and having an impedance of 51.5 ohms is constructed as above described and gives complete broad band performance up to 2 megocycles. The critical dimensions of the inner and outer conductors, theinsulators,

and X-slugs of, such a line areas follows:

Outer conductor:

outer diameter 1.625 inner diameter";; ;l; i 1.527

Inner conductori outer diameter i6 64 inner diameter .614

Insulator pins: V V

diameter .323

length --.56T

X-slug:

' outer diameter .512 inner diameter 375 length 1875 Width of wide channel -;62O

Depth of wide channel .041

Width of narrow "channeL'" L -I- Q Q .333

there lhas been :proviqeman' .electr' al .coag'r'ial transmission line'ihaving spacing mblies so constructed and arranged as tojprovidetheeletrical transparency required for RF wave transmission over the entire frequency range of the line size in use. Obviously, the invention is not restricted to the particular embodiment thereof herein shown and described.

What is claimed is:

1. An electrical transmission line comprising hollow, cylindrical inner and outer conductors and spacing assemblies radially disposed within said outer conductor at spaced points throughout its length for maintaining said conductors in concentric relation, each of said assemblies comprising a short cylindrical conductor member slidably mounted within said inner conductor with its outer surface in engagement with the inner surface of the inner conductor, said member having a circumferential channel formed on its outer surface intermediate its ends and radially disposed insulating pins extending through the Wall of said inner conductor with their inner ends projecting into said channel and engaging said member and their outer ends engaging the inner surface of said outer conductor, the depth of the channels formed in the cylindrical members of said insulating assemblies being such as to equalize the transmission line characteristic impedance at an insulating assembly with the characteristic impedance of said transmission line in the air space portions between insulating assemblies, whereby the characteristic impedance of the transmission line will be constant.

2. A transmission line, as set forth in claim 1, wherein said member is made of the same material as said inner conductor and said insulating pins are cylindrical and are made of polytetrafluorethylene.

3. A transmission line, as set forth in claim 1, wherein said .insulating pins are cylindrical and have concave inner ends and wedge-shaped outer ends, said concave ends having a curvature substantially the same as the curvature of the bottom of said channel formed on said member to provide intimate physical contact therewith, thereby excluding all air therebetween and said wedge-shaped outer ends provide intimate physical contact with the inner surface of the outer conductor along longitudinal sections thereof.

4. A transmission line, as set forth in claim 1, wherein the side walls of said channel are perpendicular to the longitudinal axis of said member and abut the side walls of said insulating pin thereby preventing longitudinal movement of said member relative to said inner conductor.

5. An electrical transmission line comprising hollow cylindrical inner and outer conductor and spacing assemblies radially disposed within said outer conductor at spaced points throughout its length for maintaining said conductors in concentric relation, each of said assemblies comprising a short cylindrical conductor member mounted within said inner conductor with its outer surface in tight engagement with the inner surface of the inner conductor, said member having a wide circumferential channel formed on its outer surface intermediate its ends and a narrow circumferential channel formed on the bottom surface of said wide channel intermediate its ends, and radially disposed insulating pins extending through the wall of said inner conductor with their inner ends projecting into said narrow channel and engaging said member and with their outer ends engaging the inner surface of said outer conductor, the depth of the narrow channels formed in the cylindrical members of said insulating assemblies being such as to equalize the transmission line characteristic impedance at an insulating assembly with the characteristic impedance of said transmission line in the space between insulating assemblies, whereby the characteristic impedance of the transmission line will be constant, the depth and width of the wide channels being such as to provide an air gap surrounding the inner ends of said insulating pins of sufiicient size to introduce an inductance in series with each of said pins of sufiicient value to cancel the residual capacitive reactance due to the particular construction of said insulator assemblies, whereby said transmission line will be electrically transparent and give broad band performance through the usable frequency range of the size line in use 6. A transmission line, as set forth in claim 5, wherein said member is made of the same material as said inner conductor and said insulating pins are cylindrical and are made of polytetrafluorethylene.

7. A transmission line, as set forth in claim 5, wherein said insulating pins are cylindrical and have concave inner ends and wedge-shaped outer ends. said concave ends having a curvature substantially the same as the curvature of the bottom of said narrow channel formed on said memher to provide intimate physical contact therewith, thereby excluding all air therebetween and said wedge-shaped outer ends provide intimate physical contact with the inner surface of the outer conductor along longitudinal sections thereof.

8. A transmission line, as set forth in claim 5, wherein the side walls of said channels are to the longitudinal axis of said member with the side walls of said narrow channel abutting the side walls of said insulating pins to prevent longitudinal movement of said member relative to said inner conductor.

LEWIS A. BONDON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,218,921 Mueller Oct. 22, 1940 2,437,482 Salisbury Mar. 9, 1948 

